It is well known that vortex generators can improve flow over an aerodynamic surface by energizing the boundary layer at the aerodynamic surface. The aerodynamic surface defines a flow boundary, and thus dynamically guides flow, such as a duct wall or aircraft surface.
The vortex generators provide a swirling flow mixing action that trails downstream of the vortex generator. Small vortex generator vanes are commonly seen on the upper, suction surfaces of wings of commercial aircraft. Vortex generators are commonly used in sets across an aerodynamic surface, such as a row across a wing, or around the interior of a duct having a bend with a low-angle diffuser.
In this disclosure, the term “vortex generator” refers to a flow mixing device placed on or adjacent to an aerodynamic surface with the objective of preventing flow separation between high speed stream flow and the boundary layer, which can be stall gas.
In the technical literature and some patents relating to burners, there is also mention of a vortex generator. The vortex generator in this case a very different device with a different objective. The burner vortex generator is a very high drag baffle plate oriented directly across the flow, in essence a multi-nozzle device with substantial solidity which squirts flow into a spiral. This device, having high solidity, severely restricts flow as well as developing an intense swirl necessary for proper burner flame holding and burner function. This is a totally different device than the vortex generators discussed in this patent, although in American English and American industrial practice, the name is unfortunately the same as the topic of this patent.
Many types of vortex generators for boundary layer control have been devised. The most widely used vortex generator consists of a relatively small vane extending a short distance out from a much larger aerodynamic surface. In the usual case, the chord of the vane (line drawn between the vane leading and trailing edge) extends at a slight angle to the direction of flow expected during aircraft flight conditions where a flow separation might occur if the vortex generator was not present. These simple vortex generators are economical, have well-known performance characteristics, and are all combined with exceptionally low drag. They are suited for improving flow in relatively weak adverse pressure gradients. However, being of small size and fastened directly to the aerodynamic surface, these vortex generators can become wholly immersed in a thickened boundary layer or stalled flow accumulations, resulting in ineffectiveness. They must thus be placed where energetic free stream flow will be dependably incident against the aerodynamic surface, which usually is well upstream of where the benefit of their increased flow mixing is desired. Despite these severe limitations, the advantages are insurmountable in many weak adverse pressure gradient applications. No other vortex generator type is in as wide use.
Many other types of vortex generators have been devised and tested. A classic discussion of experiments and data appears in G. B. Schubauer and W. G. Spanberg: “Forced Mixing in Boundary Layers,” Journal of Fluid Mechanics, 8 (1960). Both authors were with the U.S. National Bureau of Standards. Maintaining low drag is a major concern in this study.
An excellent introduction to diffusers may be found in “The Design of High Efficiency Turbomachinery and Gas Turbines,” Chapter 4, by David Gordon Wilson, The MIT Press, Cambridge, Mass., Third Printing, 1988.
A review of some U.S. patents concerning vortex generators follows:
Van der Hoeven in U.S. Pat. No. 4,655,419 “Vortex Generator” (1984) describes a vortex generator vane size relative to the boundary layer thickness, shape, and application of a curved row of vortex generating vanes to a particular jetliner aircraft wing geometry.
Bruynes, in a historic vortex generator patent, U.S. Pat. No. 2,558,816, “Fluid Mixing Device” (1947) discloses vanes similar to the van der Hoeven vortex generators, applied to improve flow in ducts, fans, diffusers, and wind tunnels. These vanes begin on a wall of the diffuser where boundary layer or stall gas is present. Vanes in diffusers have minimal effect if placed within such areas where rapidly thickening boundary layers are present. Specifically, the direction of gas flow is not always possible to ascertain in boundary layers; thus the direction of vane alignment cannot be properly chosen.
Hoadly in U.S. Pat. No. 2,650,752 “Boundary Layer Control in Blowers” (1949) discloses several types of vortex generator vanes installed in fans, on struts in ducts, diffusers, and on aircraft wings and control surfaces. In particular T-shaped vortex generators are disclosed, which T-shaped vortex generators do not have toward and away wall fluid diverting components.
Alford in U.S. Pat. No. 2,844,001 “Flow Straightening Vanes for Diffuser Passages” (1953) discloses in a turbojet outlet annulus and duct various vanes whose length is a partial passage width. These vanes straighten flow to recover swirl energy to increase thrust, and also induce vortices.
Birch in U.S. Pat. No. 4,298,089 “Vortex Generators for Internal Mixing in a Turbofan Engine” (1979) discloses vortex generators stated to reduce mixing noise of two separate flows as they merge, one cold flow from the fan and the other hot flow from the core engine. The vortex generators are mounted at the back of the core engine nozzle rim just upstream of the point of the flow confluence.
Laskody in U.S. Pat. No. 4,217,756 “Vortex Mixers for Reducing the Noise Emitted by Jet Engines” (1977) discloses stacked airfoils mounted on a radial spine that attempted to mix a hot core and cold annular flow within a nozzle. These are located just downstream of the confluence of the two streams, all in a quest for lower noise.
Ealba in U.S. Pat. No. 4,971,768 “Diffuser with Convoluted Vortex Generator” discloses a circumferential strip with, for vortex generation, a convoluted downstream edge. The application is in a pipe leading toward a diffuser.
I have experimented with this specific shape of diffuser. I have found that the convoluted downstream edge causes deflection along the circumference of the convoluted downstream edge; it does not cause efficient deflection towards and away from the diffuser walls so as to have an efficient effect on boundary layers downstream of the disclosed diffuser.
In what follows, I describe a vortex generator system that is especially useful on an aerodynamic surface that includes along its length a strongly adverse pressure gradient, such as a diffusing flow passage. A diffusing flow passage is one in which the pressure gradient increases in the direction of flow. Examples of a diffusing flow passage in a turbomachine are those diffusers found downstream of blade sections, near and in combustors, between or within compressor blade sections, and the flow path through an axial or radial compressor. Portions of turns or bends may be diffusing. In some diffusing flow passages, there may be sections where the static pressure locally stays the same or decreases in the direction of flow. However, in the diffusing flow passages here considered, for the duct as a whole, the static pressure increases in the direction of flow.
In the following discussion about diffusing flow passages, the term coefficient of pressure rise, or Cpr, is defined as:
Cpr=static pressure rise through the diffuser/dynamic pressure at the low pressure location of reference.
This low pressure reference is usually chosen at the diffuser smallest area or the diffuser inlet. The Cpr is always between 0 and 1.
The “diffuser effectiveness” is defined as the actual static pressure rise in the diffuser divided by the static pressure rise that would ideally occur in the absence of any flow losses for the duct area change of the diffuser. The diffuser effectiveness is also always between 0 and 1, but is always greater than the Cpr.
Discovery
I have discovered that the proper introduction of comparatively high drag vortex generators inhibits flow separations in areas of strong adverse pressure gradient on an aerodynamic surface where flow separations would otherwise occur. Particularly, in compact diffusers with an effectiveness of up to about 0.90, the high drag vortex generators can produce a net improvement in flow efficiency for a given diffuser area ratio, despite the high drag. This result appears to be counterintuitive and unexpected, given the high drag penalty of the vortex generator itself.
“Strong adverse pressure gradient” is defined in this application as a decelerating flow within a duct sufficient to produce losses within the incoming flow of kinetic energy of the gas exceeding 10%. Such pressure gradients are commonly found in diffusers having divergences exceeding 8° with area ratios (Outlet/Inlet) exceeding 1.75.
A “high drag vortex generator system” is a device for exchanging momentum between high speed flow and slow boundary layer flow in the high speed or free stream gas flow found away from the walls of a flow passage or the walls of a duct with a strong adverse pressure gradient. “High drag vortex generator” is defined as a vortex generators placed in a diffuser with at least one vane of each vortex generator substantially within the free stream with an effectiveness no greater than 0.90. Most of these high drag vortex generators will be in the form of intersecting vanes, often cruciform or ladder shaped.
“Free stream” or “high speed gas flow” is defined in this application as that portion of the gas flowing between a duct wall and the central portion of a duct that has a speed of 0.65 or greater than the average speed across the duct.
A “turbomachine with high speed gas flow” is defined for this patent as a gas flow housing or duct enclosing a spinning, dynamically acting impeller, that produces or consumes power, and a maximum gas velocity averaged through any plane across the flow through the machine with a bladed section of at least Mach 0.10 or a pressure ratio of highest pressure divided by lowest pressure of at least 1.1.
In the disclosure that follows, certain of my high drag vortex generator systems in diffusers appear similar to or are identical with low drag vortex generator systems in diffusers utilized in the prior art. It will be understood that in these instances, the prior art never considered the placement of these vortex generators in diffusers having a high drag environment with strong adverse pressure gradients.
In so far as the prior art does not disclose this type of high drag vortex generator alone or in a system in the turbomachinery environment, invention is claimed.
Further Background
In addition, a higher area ratio is made possible per given length of diffuser utilized. It is well known that diffusers with small included angles between walls, about 6 to 7 degrees, produce the highest effectiveness of all, but are rarely feasible because of their excessive length. Thus, the vortex generators of this invention will see most application to improving diffusers of the usual case, with their larger angles of 8 degrees and over, and accompanying high adverse pressure gradients.
Stated in other terms, the ratio between inlet and outlet on conventional diffusers seldom exceeds 1 to 2.5 or 2.6. In utilizing some of the following constructions of this disclosure, I am able to exceed these limits and approach ratios in the range of 1 to 3.0.
It is to be understood that the vortex generator of this invention differs from those vortex generator vanes of conventional design and that are commonly affixed to the wings of aircraft in at least three respects.
First, wings normally operate with relatively high efficiency aerodynamic surfaces. These high efficiency aerodynamic surfaces have relatively thin boundary layers and only mild adverse pressure gradients on the aft portions of wings and similar aerodynamic surfaces on aircraft tails. In passing through the air, such wings may have loss coefficients of just 1 or 2 percent at low angle of attack, excluding drag used to create lift. Diffusers do not include high efficiency aerodynamic surfaces.
Second, the range of flow direction of the boundary layer flow is predictable during normal flight. This is to be contrasted with turbomachines having diffusing flow passages and their sometimes the thick boundary layers and potentially unstable accumulations of boundary layers gases, in which direction of flow can be more than 45 degrees to the direction of free stream flow, often in varying and unpredictable angles.
Third, such vortex generators are fastened directly to the aerodynamic surface and are typically within a distance from the aerodynamic surface of less than 3 percent of the length, or chord, of the aerodynamic surface. This is to be distinguished from the device herein described. I attach the vortex generator vanes to a mount distanced from the aerodynamic surface that positions the vortex generator vanes at a spatial separation from the aerodynamic surface.
Fourth, conventional vortex generators usually have the junction of the vanes and mounting, and all or a substantial part of their surface area, within the low velocity boundary layer. In the low velocity boundary area, such vortex generators are low drag. My high drag vortex generator system has reduced surface area in the boundary layer, and the attachment of vanes to their mounting at a distance from the wall in the high velocity area or free stream area. This is an advantage if the boundary layer is unstable or if boundary layer flow direction is uncertain or variable.
Fifth, conventional vortex generators do not have structure and vanes with multiple or structurally redundant attachments to the flow passage walls or structure. The high drag vortex generators of this invention do.
Sixth, the high drag vortex generators may enclose the area through which the gas flows. Conventional vortex generators do not.
Most importantly, the vortex generator of my invention is not foreseen as suited to be mounted onto any substantial span of the wing of an aircraft, replacing conventional vortex generators in the usual case, because of high drag, weight, and complexity.